I can’t remember how I found this, but LibraryThing is a site where you can keep (and share) a record of the books you own. You can rate, review and tag them, find other people with the same books, get recommendations, discuss books and all that kind of webby goodness.
I just started an account to try it out and added a few books that came to hand, and quickly found it was strangly addictive. Initially I thought I might just use it to keep a record of books I was reading at the moment, but then I went and imported a list of books which I’ve bought from Amazon or told them I own, so that blew that plan. And then I added a few other favourites and books I thought were interesting. Pretty quickly, my library reached 95 titles. At this point I really have to make a decision whever I intend to do this seriously. If you add more than 200 titles, you have to pay a subscription ($10/year or $25 for life), and I’d reach that extremely quickly if I started trying to catalogue every book I own. Or even a decent selection.
I’m still not entirely sure what the point is, but as I say, I found it surprisingly addictive. And brilliantly simple to use, particularly if you have the book (and so the ISBN number) to hand.